The present invention relates to methods and apparatus for protecting well casings in high temperature process wells and more particularly to methods and apparatus for maintaining the integrity of a high temperature protective cement sheath in such wells.
Casings are normally cemented into wellbores for a number of well-known reasons. For example, in addition to providing support for the casing, a cement sheath prevents fluid communication between different horizons through which the borehole passes. In wells used for high temperature processes such as underground coal gasification, the cement sheath serves another important function. The sheath protects and insulates the metal casing from the high temperatures involved. In the high temperature zone, the surrounding material, for example, coal, is normally burned away so that the common functions of the cement sheath are lost and the high temperature protection becomes the only function.
Most metal tubular goods melt or scale at temperatures of 1500.degree. to 2200.degree. F. Such materials lose substantial strength above 900.degree. F. Refractory cements, on the other hand, are available which can withstand process temperatures in excess of 3000.degree. F. A sheath of such cement can, therefore, provide good high temperature protection to the metal tubular goods if the sheath remains intact on the casing.
A number of factors tend to destroy cement sheaths in high temperature process wells. As noted above, the surrounding material is usually burned away after a combustion process has begun. For example in a UCG process, an oxygen-containing gas is typically injected at the bottom of a coal seam and the coal is ignited at this point. As combustion proceeds, a burning rubble pile is formed in the lower portion of the zone with the gas injected into the lower portion of the rubble pile. As the coal falls away from upper portions of the zone, all of the original formation support of the cement sheath is lost. The sheath is then exposed to the highest temperatures of the process without any mechanical support or heat shielding by the original coal formation. As the injection well heats up to process temperatures, the metal casing itself expands at a higher rate than the cement sheath. This differential expansion places the cement sheath in tension, both axially and radially. Due to the known lack of tensile strength of cement materials, the sheath tends to crack and fall away from the casing which is then burned or melted through by the process temperatures.
It is apparent that the cement sheath would not normally be destroyed by process temperatures except in the high temperature zone itself. Likewise, the metal casing would tend to be destroyed only within the zone. Even after loss of these portions of the casing, it is clear that air or other combustion gases could be injected into the zone. However, it is considered very desirable, and possibly essential, to most combustion processes that the injected gases enter the combustion zone at the lowest point possible. For this reason, it is desirable to provide a reliable cement sheath around the injection casing.